1. Field Of The Invention
The invention relates generally to DC-to-DC power converters and, more particularly, to power converters particularly adapted to zero-voltage resonant transition switching.
2. Description Of The Prior Art
Switching power supplies for converting DC voltages from one voltage level to another are well known in the art. Topologies such as buck switching, boost switching and buck-boost switching are also well known. However, when conversion from a relatively high DC voltage to a relatively low DC voltage is required, the efficiency of the prior art supplies suffers. This is particularly true of voltage converters operating at a frequency of 1 MHz or higher, where switching losses become significant.
FIG. 1 is a circuit illustrating a prior art buck (or forward) converter. During normal operation of the conventional buck converter, closing the semiconductor switch S.sub.1 impresses the difference of the input voltage and output voltage across inductor L.sub.1, causing the current in the inductor to increase and charging the output capacitor C.sub.s while also delivering current to any load connected to the output. When switch S.sub.1 is turned off, the voltage at the junction between the switch and the inductor decreases until diode D.sub.1 is forward-biased. Current then flows through the diode and the inductive element with a decreasing amplitude, until the switch S.sub.1 is again closed and the cycle repeats.
FIG. 2 shows a prior art circuit for a buck-boost converter, using a transformer L for isolating the input and output voltages. The buck-boost converter uses the same number of circuit components as does a buck or boost converter and permits attaining output voltage magnitudes that are either greater or less than that or the converter's input voltage. However, this versatility in voltage gain is attained at the expense of greater switching and diode currents than those of the basic buck or boost converters. This results in substantially higher commutation losses. Further discussions of the basic DC-to-DC converters are given in R. P. Severns and G. Bloom, "Modern DC-to-DC Switch Mode Power Converter Circuits", Van Nostrand Reinhold Company, Inc., 1985.
In U.S. Pat. No. 4,618,919, "Topology For Miniature Power Supply With Low Voltage and Low Ripple Requirements", invented by H. C. Martin, Jr., assigned to the assignee of the present invention, a dual transformer switching topology of the forward converter species provided isolation of the input and output power terminals as well as voltage transformation. However, this invention was of the interleaved flyback converter type, which requires a large number of components as compared to the simple flyback converter. Further, this converter may also be subject to commutation switching losses at high frequencies.
In a paper entitled "Zero-Voltage Switching In High Frequency Power Converters Using Pulse Width Modulation", by C. P. Henze, et al, IEEE Applied Power Electronics Conference (APEC) Record, pp. 33-40, February, 1988, a zero-voltage switching technique employing resonant transition during a switching interval of short but finite duration, for application to conventional power converter topologies, offered efficient operation at very high switching frequencies while retaining the fundamental characteristics of the conventional topology. Thus, power transistor parasitic capacitor switching losses are eliminated, and the voltage stress of the power transistor during changing state was not increased. These features were obtained at the expense of increased conduction losses and the requirement for synchronous rectification.
The present invention provides a single ended DC-DC power converter, which may operate with zero-voltage resonant transition switching at very high switching frequencies (1 MHz and greater). Only one magnetic element is required, which acts as both an inductor and a transformer. The turns ratio may be varied to obtain a desired voltage conversion ratio. Control-to-output characteristics are identical to conventional power converter topologies. The circuit inherently allows for zero-voltage resonant transition switching and magnetic isolation without making special demands on the magnetic element. It provides efficient, high-frequency operation and output isolation while using constant frequency pulse width modulation for control.